It has been recognized by clinical experience and laboratory study that the rotator cuff muscle tendon complex has a functionally important role in generating load and maintaining stability of the glenohumeral joint. The purpose of this study is to continue the investigation of the rotator cuff complex by focusing on two general hypotheses: a) that the rotator cuff muscle complex contributes significantly to shoulder strength; and b) that the rotator cuff tendons are exposed to stress levels which can potentially result in tissue failure. The project has four Specific Aims, two associated with each hypothesis. Relative to the first hypothesis, Specific Aim 1 is to investigate the contribution of the rotator cuff muscle complex to shoulder strength for selected functions and joint positions by using the developed analytical model. Specific Aim 2 is to examine the influence of the type of cuff tear on shoulder strength by measuring the strength of patients with cuff pathology and associated pain, before and after injection with local anesthesia. Postulates based on the effects of tear size and number of tendons involved will be tested. In examination of the second major hypothesis, Specific Aim 3 is to evaluate the structural and material properties of the rotator cuff tendons. Mechanical testing will be performed to determine the structural strength and failure mode of the supraspinatus, infraspinatus, and subscapularis tendons. Specific Aim 4 is to calculate the stress distribution in the supraspinatus tendon for selected loading conditions. The information gathered from this study is intended to provide a better understanding of the nature of rotator cuff pathology and its clinical significance.